The invention is directed to luminescent materials based zinc silicate activated with manganese (Zn.sub.2 SiO.sub.4 :Mn), as well as a process for producing them.
Luminescent materials are solid substances which are capable of emitting light after absorbing energy (e.g. daylight, ultraviolet light, rontgen rays (X-rays) or corpuscular rays), which emitted light generally exhibits a greater wave length than that of the absorbed ray. The light radiation occurs during the excitation itself and at various lengths of time thereafter, which can be from milliseconds to hours.
Luminescent materials find expanding use, such as for example, in lighting engineering to increase the energy yield in mercury lamps, in X-ray diagnostics, or in the coating of television tubes to make visible the modulated electron beam. Besides they are used as coatings for ceramic products, especially in glazes for ceramic tiles.
Only a few inorganic compounds are capable of luminescence. Generally the luminescence capability is first acquired through an activation, for example, by building small amounts of ions (activators) foreign to the crystal lattice into the crystal lattice (host lattice) of the inorganic compound (base material). Therefore in the activator containing luminescent materials it is a matter of crystallized compounds in which a few cations of the base materials are replaced by foreign cations. The built in cations with the surrounding band of anions thereby has been conceived as the so-called luminescent centers which are responsible for the characteristic absorptions and emissions.
As base materials for the luminescent materials of significance above all are salts of the alkaline earths and their side group elements zinc and cadmium such as phosphates, silicates, borates, aluminates and oxides. As activators there are preferably used the rare earths and heavy metals, such as manganese, lead, tin, or antimony.
The luminescent materials generally must be very pure and have a crystal lattice as free as possible from disturbances since impurities in the starting materials act as quench centers and reduce the luminescent intensity of the luminescent materials. Lattice defects act similarly. The luminescent capability is damaged in the mechanical comminution of the compounds and in the extreme case can be completely destroyed.
The previously known luminescent materials have the further disadvantage that most of them are not temperature stable and disintegrate in many fritted glasses. Therefore they are not suitable as pigments in glazes for ceramic tiles which require firing temperatures of over 700.degree. to 800.degree. C.
Together with manganese activated zinc orthosilicate (Zn.sub.2 SiO.sub.4 :Mn), which occurs in nature as willemite, there is a known luminescent material which upon excitation with short wave ultraviolet light (254 .mu.m) emits in the green region. There must be employed very pure starting compounds in the form of zinc oxide, silica and manganese carbonate for the synthetic production of these luminescent materials. These additions increase considerably the expense of the final product. Besides the luminescent materials must not be ground after the last calcining, since otherwise the luminescing power is reduced.
There are known from European Pat. No. 15382 luminescent materials based on zinc silicate dosed with manganese (Zn.sub.2 SiO.sub.4 :Mn) in which a portion of the silicon is substituted by ion pairs from Group III A and Group V A of the periodic system of elements. These luminescent materials also must be produced from highly pure compounds by calcining, whereby the final product is sensitive to grinding and not resistant to the majority of the customary glazes.
Therefore, it was the problem of the present invention to develop luminescent materials based on manganese activated zinc silicate (Zn.sub.2 SiO.sub.4 :Mn) which can be produced at practically the same light intensity from commercial starting components having a lesser degree of purity, which are not sensitive to grinding and are as temperature stable and chemically stable to ceramic glazes as the known luminescent materials. Besides there is needed a process for the production of these luminescent materials.